What process occurs when the core temperature of a cooling white dwarf drops low enough, turning the matter into a rigid structure?

Answer

Crystallization

As the internal temperature drops significantly during the long cooling phase, the carbon and oxygen atoms in the core settle into a rigid, crystalline structure, a phase sometimes likened to a giant diamond.

What process occurs when the core temperature of a cooling white dwarf drops low enough, turning the matter into a rigid structure?
starsstellar evolutionastrophysicswhite dwarfBlack Dwarf